| Asset Reality | |
| Assetcool | |
| Assetminder | |
| Assetware Technology | Hertfordshire |
| Assetz SME Capital Ltd | |
| Assigments Planet | |
| Assign-IT Ltd | Hertfordshire |
| Assimila Ltd | |
| Assimilated International | |
| Associated Knowledge Systems Ltd | |
| Associated Security | |
| Association for the Understanding of Artificial Intelligence | |
| Association for UK Interactive Entertainment | |
| Association of Commonwealth Universities | Greater London |
| Association of Teachers of Mathematics | Derbyshire |
| Assured Data Protection Ltd | |
| Assured Digital Technologies Ltd | |
| Assured Platform | |
| Assyst Bullmer Ltd | West Yorkshire |
| Assyst Software | Wiltshire |
| ASTA Technology UK Ltd | |
| Astec | East Sussex |
| Astech Consultants Ltd | Warwickshire |
| Astech Projects Ltd | Cheshire |
| Astel Ventures | |
| Astex Pharmaceuticals | |
| Aston Particle Technologies | |
| Aston Solutions | Cornwall |
| Aston University | West Midlands |
| Aston University - Computer Science | West Midlands |
| Aston University - Mathematics | West Midlands |
| Aston Ventures | |
| Astoriom | |
| AstraZeneca | |
| Astrea Bioseparations Ltd | |
| Astro Communications Ltd | Kent |
| Astro Studio | |
| AstroAgency | |
| Astrohosts | Cornwall |
| AstronauTx | |
| AstroPay | |
| Astute Electronics Ltd | Hertfordshire |
| Astute Graphics | Herefordshire |
| Astute Media | West Midlands |
| Astute Technology | Essex |
| Astwellsoft | |
| ASWeb | |
| Asylum Interactive Ltd | Aberdeenshire |
| AT Engine Controls | |
| AT Ink Cart | |